The present invention relates to the field of communications in general and more particularly, to determining the geographic location of a mobile terminal.
Wireless communication systems (networks) are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
It is desirable, and in certain places mandated by law, that mobile telecommunication network providers be able to determine an approximate geographical location of a mobile terminal, such as, for example, an actively communicating cellular telephone.
A variety of mobile terminal location techniques have been proposed. These location techniques include uplink signal location, downlink signal location, Global Positioning System (GPS) based approaches, assisted GPS approaches combining communication signals and GPS signals and approaches based on digital television signals. For “uplink signal” location techniques, the mobile telecommunications network is typically configured to determine where the mobile terminal is located based on ranging measurements associated with one or more uplink signals. These uplink signals are transmitted by the mobile terminal and received by a number of receivers having known locations, such as, for example, cellular telephone base stations (BSs). For the “downlink signal” location techniques, the mobile telecommunications network is typically configured to determine where the mobile terminal is located based on ranging measurements associated with the reception, by the mobile terminal, of downlink signals from a number of transmitters having known locations. The “uplink signal” and/or “downlink signal” location techniques may be based on, for example, an Enhanced Observed Time Difference (E-OTD) technique.
FIG. 1 illustrates a conventional terrestrial mobile (wireless) telecommunications network 20 that may implement any one of a variety of known wireless communications standards including uplink and downlink signals. The wireless network may include one or more wireless mobile terminals 22 that communicate with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO 28 and may serve thousands of wireless mobile terminals 22.
The cells 24 generally serve as nodes in the network 20, from which links are established between wireless mobile terminals 22 and a MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel that may be used for downlink transmission (network to mobile) of cell identification and paging information. The traffic channels carry the voice and data information. Through the network 20, a duplex (downlink and uplink) radio communication link 30 may be effected between two wireless mobile terminals 22 or between a wireless mobile terminal 22 and a landline telephone user 32 via a public switched telephone network (PSTN) 34. The function of the base station 26 is commonly to handle the radio communications between the cell 24 and the wireless mobile terminal 22. In this capacity, the base station 26 functions chiefly as a relay station for data and voice signals. It is also know to provide mobile telecommunications networks in which the base stations are satellites, having associated coverage areas, rather than terrestrial base stations. An assisted location service 36 may be associated with the communication network and used to calculate the position of the mobile terminal.
The GPS location approach generally uses location services not associated with either the uplink or downlink signals used in the mobile telecommunications network. In a typically GPS application, the GPS receivers collect and analyze ranging measurements from signals transmitted by GPS satellites having known locations.
As illustrated in FIG. 2, GPS is a space-based triangulation system using satellites 42 and GPS control computers 48 to measure positions anywhere on the earth. GPS was first developed by the United States Department of Defense as a navigational system. The advantages of this navigational system over land-based systems are that it is not limited in its coverage, it provides continuous 24-hour coverage, which may be highly accurate regardless of weather conditions. In operation, a constellation of 24 satellites 42 orbiting the earth continually emit a GPS radio signal 44. A GPS receiver 46, e.g., a hand-held radio receiver with a GPS processor, receives the radio signals from the closest satellites and measures the time that the radio signal takes to travel from the GPS satellites to the GPS receiver antenna. By multiplying the travel time by the speed of light, the GPS receiver can calculate a range for each satellite in view. Ephemeris information provided in the satellite radio signal typically describes the satellite's orbit and velocity, thereby generally enabling the GPS processor to calculate the position of the GPS receiver 46 through a process of triangulation. It is known to include a GPS receiver 46 in a mobile terminal 22 to provide position location functionality to the mobile terminal 22.
The process of monitoring GPS signals or base station signals may be significantly affected by environmental factors. For example, GPS signals or base station signals that may be easily acquired in the open typically become harder to acquire when a receiver is within a building, a vehicle, and/or under foliage.